Imaging and temperature sensing systems for inspecting equipment condition and operation in the interior of high temperature process equipment has long been needed to monitor for when cleaning or other maintenance is required, or to detect when incipient failures are expected so corrective steps can be taken to correct the problem before it becomes an actual failure. Such equipment is needed for use in the interior of a great variety of high temperature process equipment such as furnaces, boilers, gasifiers, process heaters, ducts, hot gas filtration systems, electrostatic precipitators, and ash hoppers, and also of equipment that operates at intermediate temperatures in the region above 500° F.
One illustrative example of equipment in which such imaging and temperature sensing systems would be very useful is in catalytic hydrogen reformers. Catalytic hydrogen reformers are often used in petroleum refineries as source of hydrogen for use in hydrotreating to remove sulfur from heavy fractions and high sulfur crude. In the hydrogen reformer, a stream of light refinery off-gas plus natural gas and steam is passed through catalyst filled tubes inside a furnace to carry out a reaction that converts the methane and refinery off-gases into hydrogen and carbon dioxide. The catalyst (most commonly nickel) enables the reaction to proceed nearly to completion at very high temperatures—typically around 850° C. The high temperature requires the use of tubes composed of exotic, very expensive alloys. However, even these alloys can be damaged by exposure to excessively high temperature, so it is imperative to maintain the tubes within a safe temperature range. When operated properly, a hydrogen reformer furnace can operate for several years before tubes need to be replaced and catalyst regenerated. Overheating can lead to forced outages and can require premature tube replacement, which is extremely costly—both in labor and equipment costs as well as in lost production.
At the present time there is no commercial technology available that can continuously monitor tube temperature profiles within an operating hydrogen reformer furnace. Infrared pyrometers are capable of measuring tube temperatures, but provide only a point measurement. Therefore, to develop a complete map of tube temperature distributions, it is necessary to make point measurements at a very large number of locations and assemble those measurements into a map—a very cumbersome and time-consuming process. Also, infrared pyrometer measurements can produce inaccurate results due to two effects. One is that the emissivity of the tube surface must be estimated in order to allow temperature to be inferred from a measurement of radiant intensity. Often the emissivity is not known since it can change over time as scale forms on the outside of the tubes. Also, tube emissivity depends to some extent on the tube temperature, creating a “catch-22” situation where the tube temperature must be known in order to estimate the emissivity, but the emissivity must be known in order to convert measured radiant intensity into temperature. A second issue that must be taken into consideration is that not all of the radiation emanating from the tube surface results from thermal emission—some is reflected or scattered light that originates at other locations within the furnace. Most hydrogen reformers are lined with refractory whose surfaces are much hotter than the reformer tubes, so—although the emissivity of the refractory may be lower than that of the tubes—the thermal radiation emitted by the refractory walls can be quite significant and must be accounted for in order to achieve acceptable levels of accuracy in determining the actual tube temperature from a passive measurement of the radiation emanating from the tube surface.
A video imaging system provides a means of collecting such radiant intensity information about a very large number of locations throughout the furnace interior simultaneously and continuously. Furthermore, a silicon CCD camera has sufficient linearity and reproducibility to permit quantitative comparisons of intensity on a pixel by pixel basis. One challenge in using a visible light sensitive imaging system to measure temperature, however, is that the amount of thermally emitted radiation in the visible spectrum is relatively small compared to that emitted in the infrared (except at very high temperatures), and the change in intensity for a given change in temperature is also much smaller than in the infrared. Nevertheless, modern silicon CCD cameras have very high sensitivity and very large dynamic range, so it is possible to relate image intensity to temperature in situations where the scene temperatures are above, say, 500° C. Therefore, a camera system which images the interior of a hydrogen reformer and provides a map of intensity variation over the viewed area, when combined with a separate measurement of temperature at one or a few specific points within the scene, would allow the temperature of every point within the scene to be determined.
A system that would provide operators with a continuous measurement of tube temperature distributions and display the results in the form of an image (possibly color coded to represent temperature), would be of immense value to the petroleum refining industry.